By relative volume of the known things. It’s not a guarantee, but it’s highly suggestive that the more observable instances of something, the more not yet observed instances of the same thing are out there.
There are factors that can knock that out of balance, like not having access to source code making things harder to find, but those confounding factors would hide more on the closed source side than the open source side.
I’m not sure how you can provide evidence that one thing has fewer unknown unknowns than another thing.
By relative volume of the known things. It’s not a guarantee, but it’s highly suggestive that the more observable instances of something, the more not yet observed instances of the same thing are out there.
There are factors that can knock that out of balance, like not having access to source code making things harder to find, but those confounding factors would hide more on the closed source side than the open source side.